Thermal gap control

ABSTRACT

A heat dissipating element (e.g., a heat sink) is held in an initial position closer to a heat generating structure (e.g., a microprocessor) and in a subsequent position farther from the microprocessor. A thermal interface material (e.g., a thermal grease) spans the gap, but is not held under compression, between the heat sink and the microprocessor.

BACKGROUND

To prevent microprocessors and other heat generating electroniccomponents from overheating, excess heat is sometimes conducted to aheat sink where it is dissipated. The heat sink may be mounted above themicroprocessor with a thermally conductive elastomer held in a thermalgap between the heat sink and the microprocessor. The elastomer is heldin compression between the heat sink and the microprocessor to provide agood thermal conduction path that will last for a long period.

In such an approach, the distance between the upper surface of theprinted circuit board and the upper surface of the microprocessorpackage may vary from unit-to-unit because of manufacturing tolerances.The spring device provides enough free play to accommodate such changes.

SUMMARY

In implementations of the invention, a heat dissipating element (e.g., aheat sink) is held in an initial position closer to a heat generatingstructure (e.g., a microprocessor) and in a subsequent position fartherfrom the heat generating structure. A thermal interface material (e.g.,a thermal grease) spans the gap, but is not held under compression,between the heat sink and the microprocessor.

Because no compression is applied to the thermal interface material,soft materials that have little or no strength to resist compressiveforces may be used as the thermal interface. The thermally conductivepath remains good over a long period. Variations in the thermal gap areaccommodated without imposing a compressive force on the thermalinterface material. Tight control over the size of the thermal gap ismaintained.

Other advantages and features will become apparent from the followingdescription and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a mounted microprocessor inaccordance with an embodiment of the invention.

FIGS. 2 and 3 are cross-sectional schematic side views in more detail,showing two stages in manufacturing.

FIG. 4 is a flow chart.

DESCRIPTION

As seen in FIG. 1, a microprocessor 10 (or other heat generating device)is held in a socket 12 that is mounted on a printed circuit board 14(e.g., a computer circuit board such as a motherboard) using a ball gridarray 16. A metal block or plate forming a heat dissipating element(e.g., a heat sink 18) is attached to the board 14 using supports 27, 29and is held in a position that defines a thermal gap 20 between thebottom surface 22 of the heat sink and the top surface 24 of themicroprocessor 10. (Although only two supports are shown in the figure,there are actually three or more supports arranged in a triangle or arectangle.) A thermal grease, gel, or other soft, highly thermallyconductive material 26 spans the gap 20 and defines a heat conductingpath 28 from the microprocessor to the heat sink. The grease is not heldunder compression so it remains in place and provides a good thermalpath 28 over a long period without begin squeezed from the gap.

The height 30 from the top surface 31 of the board to the top surface 24of the microprocessor varies from unit to unit because of manufacturingvariations. It is desirable for the gaps 20 in different units to be ofthe same height despite of the differences in the heights 30, in orderto provide a consistent adequate thermal path.

As seen in FIG. 2, the gap 20 is kept at a desired fixed height by anarrangement in which the four supports (pins in this case) 27, 29 aresecurely attached to the heat sink at their upper ends, pass throughfour corresponding holes 45, 46 in the printed circuit board, and aresecurely attached to the board by locking clips 42, 44 held on the lowersurface of the board. High-stiffness coiled spacer springs 52, 54surround the pins.

Referring to FIG. 3 and to the flow chart of FIG. 4, duringmanufacturing, after the microprocessor has been inserted into themounted socket, a quantity of the thermally conductive material 26 isplaced (70) on the top surface of the microprocessor or on the bottomsurface of the heat sink. The heat sink is then attached (72) bylowering it over the microprocessor and the pins are inserted throughthe holes and into the locking clips. The heat sink is pushed down(forcing the pins further into the locking clips) until the bottomsurface of the heat sink contacts the top surface. The thermal grease issqueezed from the gap as indicated by arrows 50 but remains temporarilyin the vicinity of the perimeter of the gap.

Next, referring again to FIG. 2, when the downward force on the heatsink is released, the springs force the heat sink upward. The lower endsof the pins are grabbed by the sharp edges of the holes in the lockingclips and, as the heat sink moves upward (74), the pins pull up on thelocking clips, distorting them from their original conical shapes toflat shapes that define jammed positions. The vertical distance traveledfrom the lowest position of the heat sink to the position at which theclips are jammed is relatively constant from unit to unit and definesthe gap. As the heat sink is pushed upward, the thermal grease is drawnback (76) into the gap by viscous forces and suction as indicated byarrows 60, to fill the gap and provide the desired thermal path. In thisfinal position, no compressive force is applied to thermal grease, whichtherefore is able to remain in place and provide a good thermal pathover a long period of time.

In particular implementations, the microprocessor could be a Pentium® IIprocessor or Pentium® III processor or other microprocessor, forexample, of a kind that is surface mounted for use in notebookcomputers.

Other arrangements can be used to attach the heat sink to the board,including a leaf spring or tab, a screw spring combination, an externalheat sink like clip with a spring between the heat sink and that board.

What is claimed is:
 1. Apparatus comprising a support, a heatdissipating element attached to the support, a heat generating elementattached to the support and defining a gap between the heat dissipatingelement and the heat generating element, and a viscous material spanningthe gap, the support being configured to permit adjustment of a size ofthe gap over a range from smaller sizes up to a predefined size, thesupport including a resilient element that biases the support to aposition in which the gap has the predefined size and the viscousmaterial spans the gap.
 2. The apparatus of claim 1 in which the heatdissipating element comprises a metal heat sink.
 3. The apparatus ofclaim 1 wherein the support comprises a retainer that holds the heatdissipating element in the position in which the gap has the predefinedsize.
 4. The apparatus of claim 3 in which the retainer comprises a pinheld by a gripping mechanism and the resilient element comprises acoiled spring surrounding the pin.
 5. The apparatus of claim 1 in whichthe heat generating element comprises a heat generating component and aboard on which the component is mounted.
 6. The apparatus of claim 5 inwhich the heat generating component comprises a microprocessor.
 7. Theapparatus of claim 5 in which the support holds the heat dissipatingelement on the board.
 8. The apparatus of claim 1 in which the viscousmaterial comprises a soft material.
 9. The apparatus of claim 1 in whichthe viscous material comprises a high-conductivity thermal grease orgel.
 10. The apparatus of claim 4 in which the gripping mechanismcomprises a locking clip that changes from a conical shape to a flatshape when the support is adjusted from a position where the gap has asmaller size to a position where the gap has the predefined size. 11.The apparatus of claim 1 in which the viscous material changes frombeing compressed to not being compressed when the support is adjustedfrom a position associated with a smaller gap to a position associatedwith the predefined gap size.